Hi RAF and a warm welcome to the Scale Models forum. Converting the
Airfix lifeboat should be a relatively straightforward task but the defining criteria with most of these conversions is the inherent stability of the model. Unfortunately you simply won't know if this will work without buying the kit and doing some ballast trials in the bath.
It's all to do with weights in scale but put very simply if the upper structure of the kit is too heavy as a result of the thickness of the plastic and the weight of the fittings then it will not be stable when you put the running gear in. Another consideration is the weight of ballast you need to get it down to it's marks, therefore defining how much available weight you have for the propulsion system. Imagine for instance if the model is so heavy that when you put it in the water it is already at it's normal water level than you can't add any more weight.
My gut feeling with this model is that it will be too top heavy but ultimately you will have to buy one, do some tests with the open hull and make your own decision. Another possibility is that you could remake a lot of the upper structures from scratch in a much lighter material than the original but you won't know if this is necessary until you do some tests in the bath.
As for running gear, I would go for a servo motor, which come in at about 16mm diameter, with 2mm shafts and 4mm shaft tubes, all of which are available from SHG models. I would also use the Robbe rubber tube type of coupling which are a lot lighter and smoother running than any of the available UV joints.
Have a read through my and Tiger's U-Boat conversion build threads which may give you some ideas:
http://www.scale-models.co.uk/submarines/5500-revell-1-72-viic-u-boat-static-surface-runner-build.html
http://www.scale-models.co.uk/under-construction/2410-revell-1-72nd-type-vii-u-boat.html
